The objective of this research program is to increase our understanding of the mechanisms by which the nervous system controls movements of the mandible. The technique of single motor unit electromyography will be used to study the function of the muscles of mastication in human subjects. These studies will be directed toward elucidating the role of the masseter, medial and lateral pterygoid muscles in, (1) the maintenance of the normal "rest position" of the mandible and, (2) in the generation of large forces between the occlusal surfaces of the teeth. The role of feedback from activation of receptors in the mucosa of the oral cavity on the control of the generation of these forces will also be investigated. Subjects in these studies will also include patients with the Myofacial Pain Dysfunction Syndrome. The analysis of single motor unit activity in the masseter and pterygoid muscles will provide a useful tool in investigating the etiology of the neuromuscular disorders of the masticatory system which are common in these patients. Neurophysiological studies in experimental animals will be utilized to investigate the mechanism by which the influence of cortical centers interacts with feedback from receptors in the oral mucosa to integrate the activity of the motoneurons which control jaw movement. Electrical stimulation of the cortex will be used to evoke rhythmic masticatory movements. The effect of the cortical stimulus will be observed on neurons in the trigeminal sensory and motor nuclei before and during the addition of input from intra-oral receptors.